Usually wear & tear. Most cases of chordae tendinae rupture are because of degeneration after years of routine "wear and tear" leads to thickening and retraction. Rarely, it can occur due to infection (endocarditis), either acutely, or later if fibrosis develops and distorts the chorda. If a mitral valve is extremely "floppy", i.e. Mv prolapse, the flopping leaflet may pull the chorda hard enough to tear it.